Sonar systems presently exist for use by fishermen in determining the depth of the lake bottom below their boat and also the presence and depth of fish, if any, between the boat and the bottom. Knowledge of the lake or river depth, along with a knowledge of the water temperature and fish feeding habits, are of great aid to fishermen.
Such sonar systems utilize a transducer that converts a transmitted electrical frequency pulse signal, such as 200 KHz, into a sound signal at the boat and directs the sound signal down through the water. The transducer will then receive back an echo signal from the lake bottom, the length of the passage of time between the transmittal of the sound pulse and the receipt of the echo pulse being a direct measure of the distance from the boat to the lake bottom.
The present systems incorporate a suitable distance calibration device that gives a reading to the fishermen directly in feet.
Such systems provide a radio frequency transmitter for delivering the radio frequency electrical energy to the transducer that then converts the electrical signal to a sound signal at the same radio frequency. A receiver circuit is also provided coupled to the transducer to convert the sound echo signal to electrical energy for operation of the suitable signal indication to the fishermen.
The principal problem with existing sonar systems stems from the fact that the transmitter sections and the receiver sections of the system incorporate LC tuned circuits to establish the desired frequency band of the generated and received signal of, for example, 200KHz .+-. 20 KC. However, the sound transducer incorporates impedance characteristics of its own, such as induction and capacitance, and each transducer must be carefully matched with the transmitter-receiver circuitry to give the desired frequency output. Careful frequency alignment is needed during the initial manufacture of the sonar system, and variations introduced by the fishermen during use, such as a different length or size of connector cable between the electronics and the transducer, can result in miscalibration of the unit. More important, different transducers cannot be substituted into the sonar system without returning the unit to the repair shop for realignment. Thus broken or lost transducers cannot be easily replaced. Also, separate transducers cannot be permanently mounted on separate boats with the electronics package portable from one boat to another.
Still another problem with existing sonar systems is the fact that the transmitter utilizes one LC tuned circuit to produce the desired transmitter pulse frequency and the receiver utilizes a second LC tuned circuit tuned to the frequency of the r.f. signal received from the transducer responsive to the received echo sound. Various factors such as heat and time change the frequency of tuning of these tuned circuits so that they no longer match, and the calibration of the sonar system degenerates.